1. Field of the Invention
The invention relates to paper machine clothing useful for fabrication of dryer belts, employed in the dryer section of a papermaking machine, wet belts employed in the press section of such machines and forming wires which may be used on fourdrinier and cylinder machines and more particularly relates to such fabrics made from spun yarn, multi-and monofilaments of synthetic polymer resins and including a means for chemical treatment during their use.
2. Brief Description of the Prior Art
Papermaking machines are well known in the art. The modern papermaking machine is in essence a device for removing water from the paper furnish. The water is removed sequentially in three stages or sections of the machine. In the first or forming section, the furnish is deposited on a moving forming wire and water drained through the wire to leave a paper sheet or web having a solids content of circa 18 to 25 percent by weight. The formed web is carried into a wet press felt section and passed through one or more nip presses on a moving press felt to remove sufficient water to form a sheet having a solids content of 36 to 44 percent by weight. This sheet is transferred to the dryer section of the papermaking machine where dryer felts press the paper sheet to hot steam heated cylinders to obtain a 92 to 96 percent solids content.
On papermaking machines, endless belts are employed in the various sections to carry the sheet or web of paper. There are a wide variety of forms of the endless belts, some fabricated from metal and others from textile material such as cotton, cotton and asbestos or cotton, asbestos and synthetic fibrous or filamentous materials. The selection of a given material is dependent to some degree upon the use to which the fabric will be put, i.e.; as a forming fabric, dryer felt, etc.
One form of belt which has been used extensively as a forming wire in the forming section of the papermaking machine is one fabricated from an open weave of synthetic, polymeric resin monofilaments. Such fabrics generally perform well in the forming section although there are certain limitations. For example, the resin monofilaments have an affinity for accumulating a build-up of pitch, tars and other contaminants during use. This shortens the overall life of the forming wire and requires frequent halts of its papermachine for cleaning and application of inhibiting chemicals. This shut-down cleaning may be required as frequently as on a weekly basis.
Dryer belts for use in the drying section of the papermaking machine have historically been fabricated from dryer felt fabrics. In recent years, one form of belt commonly employed in the dryer section of a papermaking machine is referred to as a "screen" and is fabricated by weaving synthetic monofilaments or twisted multi-filaments together in an open weave. Although not subjected to any form of milling, and therefore not "felts" in the original sense of the term, these screen fabrics have also become known as "dryer felts". The endless belts are generally woven flat and the ends thereafter joined to form an endless belt. The weave selected may be a two or three layer weave of synthetic yarns such as multifilament, spun or monofilament yarns.
In carrying the formed paper web through the dryer section of the papermaking machine, the felt aids in drying, controls shrinkage of the paper web and prevents cockles. The felt fabric must possess strength, dimensional stability, resistance to chemical and thermal degradation, resistance to abrasion and have a functional permeability. In recent years all monofilament structured fabrics have been developed to meet the above-described needs of a dryer felt. However, dryer felts fabricated from all monofilament fabrics, like forming wires, accumulate deposits of pitch, tar and other contaminants such as paper duct. Shut-down cleaning may be required as frequently as every 2-3 weeks and cleaning over a long period of time may become less effective as filling of the felt voids continues. This of course may be highly undesirable, resulting in a high percentage of unsatisfactory paper product.
Those skilled in the art have long appreciated that the efficiency of water removal in the wet press section of the papermaking machine is critical to overall efficiency in the papermaking process. This is because, first a large amount of water must be removed from the sheet at the presses to realize a good drying economy. Secondly, greater efficiency in water removal creates a drier and hence stronger sheet less susceptible to breaking. A large variety of clothing constructions have been proposed as papermaker's felts advantageously employed in the press section of a papermaking machine. In fact, there has been a continual evolution of clothing constructions, corresponding to improvements in the papermaking machine itself. This evolution began with the early woven felt, woven of spun yarn and then mechanically felted or fulled. A later development was found in the "Batt-on-Base" construction consisting of a woven fabric base and a batt surface attached by needling. The needled batt-on-base felts are widely used today and have been said to be the "standard of the industry". However, a wide variety of other constructions are available, including non-woven press felts and composite laminates which comprise a fabric substrate with a surface layer of a flexible, open-cell, polymeric resin foam; see for example the disclosures found in U.S. Pat. Nos. 1,536,533; 2,038,712; 3,059,312; 3,399,111; and 3,617,442. In general, the papermakers wet-press felt fabrics, like forming wires and dryer fabrics, require periodic chemical treatment or cleaning to remove debris or contaminants which accumulate during use. Certain chemical additives are also advantageously used during initial break-in periods. For example, it is common knowledge to use small amounts of a detergent applied through a full width shower on wet-press felts during the application of a felt, i.e., the pressing of water through a press roll. The use of such a detergent shower is intended for conditioning a wet-press felt to be more absorbent to water and thus aid a felt in keeping clean and draining properly. We also know from prior art that the use of free detergent dissolved in the stock water aids in pressing water from the sheet of paper being produced.
Using the concepts taught by the present invention, the surfactant is at the point of application since it is being carried in the felt itself going through the press nip.
Dispensing of small quantities of surfactant throughout the life of the wet-press felt is ideal. However, since difficulty in pressing water from paper is mostly experienced in the initial few days and during the compaction of the felt to its equilibrium caliper, the addition of surfactant during this break-in period is essential. During break-in and compaction, it is also important to keep the felt clean such that paper stock particles are not trapped within the felt causing the disruption of channels in normal felt drainage. The surfactant would act as a cleaning agent as well. It is also possible to minimize cost, and foam buildup since this method reduces amount needed because dispensing of the surfactant is controlled.
With the structured fabrics of the present invention, many of the above-described shortcomings of the prior art are removed. Dryer belts constructed according to the invention may be fabricated from an all monofilament fabric which provides for extended periods of time an exceptionally smooth surface to contact the paper sheet. As a result, relatively mark free paper product is obtained, while all of the desired advantages of an all monofilament dryer felt are retained.
Wet-press felt fabrics are broken-in more rapidly and require less frequent shut-down cleaning, thereby raising the efficiency of the papermaker's machine over periods of time. The overall operating life of the forming wires and felts is significantly increased over prior art wires and felts.
It will be appreciated that there is an extensive range of prior art descriptions in the field of papermaker's fabrics. Representative of such descriptions are those found in U.S. Pat. Nos. 2,260,940; 2,354,435; 2,748,445; 3,060,547; 3,158,984; and British Pat. No. 980,288.